The present invention relates to a process for the selective and controlled reduction of the relative permeability to water in oil-bearing formations, and more specifically to such a process whereby said reduction is achieved with the aid of successive polymer slugs, which are adsorbed to the rock surface and among themselves, creating a hydrophilic film which swells in the presence of water and strongly reduces the permeability to water. On the contrary, in the presence of oil the film keeps its original thickness, so that the permeability to oil is only slightly reduced.
The production of water associated to the production of oil has been one of the major drawbacks for oil companies worldwide. The breakthrough of the formation or injection water results in a strong decline of oil production, increasing the operation costs for pumping, treatment and discarding of huge volumes of water.
Different techniques may be used to reduce the production of water or to prevent its anticipated breakthrough, the choice of the best technique being a function of the origin of the produced water. Therefore, the analysis of the well and the reservoir should precisely determine the origin of such water.
Water coning or channeling of injected water frequently causes high water/oil ratios so that the use of flow barriers is successful. Such barriers may have a plugging effect or they may reduce permeability, that is, the systems may be respectively blocking or selective.
The plugging barriers, which block simultaneously the flow of gas, oil and water, may be formed by cements, resins, solid particle suspensions, silicates, wax, or water soluble polymers crosslinked in situ in the reservoir.
On the contrary, selective systems block mainly the flow of water, while the flow of hydrocarbons is only slightly restricted. These selective barriers are generally formed by water-soluble copolymers, which adsorb to the surface of the rock, forming a hydrophilic film, which reduces water mobility.
The preparation of selective polymer systems is a function of the mineralogy and permeability of the rock, reservoir temperature and salinity of the formation water. The kind of rock, sandstone or limestone, defines the ionic character of the polymer to be used as the first slug to contact the formation, since such first slug promotes a better adsorption of the next plug of polymer. Thus, the adsorption sites mainly negatively charged on the surface of sandstones require cationic polymers. Inversely, limestones are favored by the anionic character of polymers to enhance the adsorption ability and thus the formation of the hydrophilic film in charge of the selective reduction of water permeability.
High formation temperatures may inhibit the effect of applied treatments. Under these circumstances, the polymer tolerance may be improved by adding products such as AMPS (2-acrylamido-2-methyl propanosulfonic acid) and/or vinyl pyrrolidone to the molecular structure of the base polymer.
On its turn, salinity may inhibit the swelling of the hydrophilic film and thus hinder the effect of permeability reduction to water.
As regards the rock permeability, the higher this parameter, the thicker should be the hydrophilic film. This may be obtained through the increase in the molecular weight of the polymer (monolayer configuration), or through the piling of cationic and anionic polymers and crosslinking agents (multilayer configuration).
Further main advantages of the selective systems are:
The treatment is injected through the annulus or through the string, dispensing with the need to modify the equipment normally in the production of the well; perforation or completion rigs are dispensed with.
On the contrary, plugging treatments may require the isolation of the zone to be treated. Rigs are then required and the operation cost is considerably increased.
U.S. Pat. No. 4,409,110 teaches a process for the secondary or tertiary recovery of oil using a mixture of non-ionic and polycationic polymers. The mixture leads to a reduction in viscosity in a first pumping phase while after the injection of the mixture, the adsorption of the cationic polymer on the rock causes an increase in viscosity, so as to improve the sweep efficiency. At the same time, the observed adsorption causes the inhibition of the swelling of the clays. On the other hand, under certain circumstances of pH and temperature the non-ionic polymer undergoes hydrolysis, and becomes anionic. By contacting the cationic polymer, the anionic polymer forms a solid precipitated which would also be adequate for use in producing wells as plugging agent.
U.S. Pat. No. 4,498,539 teaches a process for the correction of permeability to water and a composition for the process, to improve the efficacy of the water injection. Such process involves the sequential injection of i) one optional aqueous slug in order to adjust the salinity of the formation water; ii) an aqueous thickened slug; iii) an aqueous entrainment fluid. The aqueous thickened slug is made up of a polyvalent metal cation, such as aluminum citrate, a gellable polymeric viscosifier such as polyacrylamide, and a hydrolizable ester suitable for the pH adjustment which is effective for the delayed reduction of the pH of the polymer slug, so as to provide the ideal condition for in situ gellification.
U.S. Pat. No. 5,079,278 teaches a gel-forming polymer aqueous composition, which is able to selectively plug high permeability zones in subterranean oil-bearing formations. The composition comprises an aqueous solution of a high molecular anionic terpolymer of acrylamide, which contains of between 5 to 95 weight % 2-acrylamido-2-methyl propane sulfonic acid, of between 2 and 95 weight % N-vinyl-N-methyl acetamide and of between 5 to 93 weight % acrylamide, and a crosslinking agent selected among the group comprising transition metal ions, phenolic resins and amino resins. The described composition forms stable gels in brines of a wide range of salinity, the gels being effective at the pH levels encountered in carbon dioxide and water injection.
U.S. Pat. No. 5,379,841 teaches the reduction or the elimination of the permeability to water by injecting a copolymer, which contains anionic and cationic portions in its structure. It is alleged that by varying the amount of types of comonomers present in the copolymer it is possible to achieve improved adsorption of the polymers to various kind of rocks, from sandstones to limestones. In spite of the alleged advantage, that the polymers may be used in various kinds of rocks, the synthesis of polymers having complex structures, which should be tested for each particular situation, is certainly time-consuming and expensive. Besides, said US patent uses high molecular weight polymers in one single layer, this being only effective for high salinity, and low to medium permeability reservoirs. In case of high permeability, the efficacy of the process using a high molecular weight polymer as a monolayer may be critically impaired.
On the contrary, the present invention may be extremely flexible by only using ready commercial products, this rendering the present process cheap and versatile, since the need of the particular formation is met by varying the number of successive polymer layers. The flexibility of the applications according to the present invention is also consequent to the various molecular weights and hydrolysis degrees of the commercial products used, which may be dimensioned as a function of the reservoir conditions (kind of rock, temperature, salinity and permeability).
European EP 0197684 teaches reducing the permeability of sandstones by injecting an aqueous solution of a water-soluble anionic polymer having a molecular weight higher than 100,000, followed by the contact of said polymer with a stabilizing fluid which comprises a water-soluble cationic polymer having molecular weight higher than 1,000. In spite of the alleged results, it should be noted that sandstones are rocks having mostly negatively charged rocks, and that the contact of said negative charges of the rock with a polymer itself negative (anionic) does not yield the desired adsorption level, so that the obtained film is of reduced thickness. In spite of the addition of the stabilizing cationic polymer, the results obtained for Residual Resistance Factor are poor (see Table III page 28 of the specification of EP 0197684) except for experiment 3, which shows good results. Said European patent does not make reference to the use of successive layers of anionic polymer and cationic stabilizer.
U.S. Pat. No. 5,789,351 teaches a process for altering the permeability of subterranean formations through the use of a gelling composition, which contains a carboxylate-containing polymer and a trivalent crosslinking agent related to zirconium. No gelling delayer is used, the delay being obtained by preparing the polymer solution in a brine where the amount of dissolved salts serve as a gelling retarder.
Thus, the patent literature teaches various procedures where i) the polymers used are weakly adsorbed by the rock and present low molecular weight, with the ensuing low values of Residual Resistance Factor (RRF), that is, the plugging of the water production zone is below the desired value, or ii) complex syntheses and expensive products are involved, with the use of the monolayer configuration. However, many times said products reveal themselves useless in formations of high salinity and high permeability.
The same way, most known processes which use a trivalent crosslinking agent somehow requires that the crosslinking reaction be delayed so that the reaction may occur in the reservoir and not in the production string.
Thus, the literature does not describe nor suggest a process for the selective and controlled reduction of the permeability to water of water producing areas in sandstones, such as described and claimed in the present application. Therefore the process of the Applicant comprises therefore the injection in a producing well, of an aqueous solution of cationic polymer where the polymer composition shows high adsorption to the sandstone rock of the reservoir, followed by an aqueous solution of anionic polymer, and optionally of a slug of cationic crosslinking agent to effect a punctual crosslinking so as to form layers but without forming gel. All the polymer slugs are separated by the injection of spacing aqueous solutions which prevent the formation of gel, and allowing the contact of polymers of different ionic nature only in the making up of the hydrophilic film. Then, further anionic, crosslinking and spacing slugs are injected, until the Residual Strength Index is attained. Such process is described and claimed in the present invention.
In case of a limestone rock, rich in positive charges, the order of addition of the polymer compositions of the invention is inverted. Thus, the injection into the production well is started by a polymer composition containing an anionic polymer, which may or may not be followed by slugs of cationic polymer or trivalent crosslinking agent, which works as a cationic layer former.
No matter the kind of reservoir rock, it is preferable that the last layer be made up of a cationic polymer or of the crosslinking agent, which works as a stabilizer.
The process for the selective and controlled reduction of the permeability to water in formations penetrated by at least one production well for the production of oil and/or gas bearing sandstone rocks according to the present invention comprises in a general way the alternated injection, into a production well, of slugs of aqueous cationic polymer solutions, followed by the injection of slugs of aqueous anionic polymer solutions and optionally slugs of trivalent cationic crosslinking agent, optionally followed by the injection of further slugs of anionic polymer solution and slug of trivalent crosslinking agent, forming layers until the desired Residual Resistance Factor is attained. The slugs of polymer solution and crosslinking agent are always separated by slugs of an aqueous solution of an alkaline salt such as NaCl or KCl.
When the formation is rich in limestone rocks, at first a slug of an anionic polymer solution is injected, followed by successive slugs of cationic polymer solution or of multivalent crosslinking agent and anionic polymer, the last layer being always made up of a crosslinking agent, so as to confer to the hydrophilic film a cationic character. All the polymer solution slugs are alternated with spacing slugs of an aqueous solution of an alkaline salt such as KCl.
In order to avoid the mixture of solutions of different polarity within the well, the slugs of aqueous cationic and anionic polymeric solutions as well as the slug of aqueous solution of the multivalent cationic crosslinking agent are separated by slugs of aqueous solutions of alkaline salts such as KCl. This serves to prevent the mixture of the said cationic and anionic polymer aqueous solutions (as well as the mixture of said solutions with the crosslinking agent) which would cause flocculation, precipitation or gel formation in the string, this bringing harm to the injection of the aqueous solutions themselves into the reservoir.
The dimensioning of the spacing slug is effected so as to prevent the occurrence of flocculation, precipitation or gel formation near the face of the well, this bringing harm to the injection, besides reducing the possible extension of the treatment.
Therefore, the spacing slug between the several slugs of aqueous polymeric solutions addresses a main need, that is, to allow that the several slugs be adsorbed to the polymer films which have been previously formed, without forming gel or precipitates, and this, at the desired treatment extension. By using spacing slugs, the procedure of the invention makes possible to dispense with the use of crosslinking retarders employed in state-of-the-art technique, such as for example aluminum citrate.
The concept of the present invention involves therefore injecting into a production well as many layers of polymer as required to attain the desired Residual Resistance Factor. That is why it is possible to state that the present process involves the selective and controlled reduction of the permeability to water, since the layers of polymer and crosslinking agent are injected into the production well according to the condition of permeability, salinity and temperature.
Thus, according to the conditions of the formation, the same polymer solution may be injected in several layers, or different polymers will be injected, forming layers, which will yield the desired effect.
In the field, the injection of the several slugs of polymer solutions is controlled by the pressure and flow rate of injection of the slugs, this being an indication of the attained reduction in permeability.
Therefore, contrary to the teachings of the patent literature, the present invention makes use of a concept which leads to the selective and controlled reduction of the permeability to water not by the plugging of the formation pores with insoluble gels but instead by the accumulation of adsorbed polymer layers, the kind of polymer to be injected as well as the number of layers being a function of the permeability, temperature and salinity of the producing formation.
Still, contrary to the gels described in the patent literature, where full crosslinking eliminates all residual charges from the formed gel, the punctual or partial crosslinking promoted by the crosslinking agent applied according to the invention leaves residual positive charges on the surface of the last layer of anionic polymer (or crosslinking agent), said charges serving for attraction as well as anchoring to the next anionic layer to be injected in the well.
According to one preferred mode of the invention, for sandstone rocks having medium to high permeability and medium to high salinity, the slug of polymer solution to be injected in the production well may be made up of a layer of cationic polymer followed by a layer of anionic polymer and crosslinking agent, followed or not by further layers of anionic polymer and crosslinking agent, so as to form various layers.
For producing formations having low permeability and low salinity the process may be much simpler: one single layer of cationic polymer may be injected in the production well, or one layer of cationic polymer followed by a layer of anionic polymer, followed or not by other layers of cationic and anionic polymer.
According to another preferred mode of the invention, as applied to formations rich in limestone rocks having medium to high salinity and medium to high permeability the slug of polymer may be made up of one layer of anionic polymer followed by successive layers of multivalent crosslinking agent and anionic polymer.
For more favorable situations, that is, low permeability and/or low salinity, one single layer of anionic polymer is sufficient.
The several process options and the corresponding compositions are chosen as a function of the salinity of the formation water as well as the permeability of the rock and temperature.
Broadly, for a production well, the process of the present invention for the selective reduction of the permeability to water comprises:
by injecting into said production well a slug of water or of cleansing organic solvent the volume of which should be close to that of the annulus or of the production string, withdrawing any damage caused to the formation by precipitation of waxes or asphaltenes;
injecting alternate slugs of aqueous cationic or anionic polymer solution, so as to form layers, each layer of anionic polymer being optionally linked to another layer of anionic polymer through a slug of cationic crosslinking agent, each slug of injected polymer aqueous solution being separated by a slug of aqueous solution, all aqueous solutions, either polymeric or not, being added of an amount of alkaline salt sufficient to prevent the swelling of the reservoir clays, and so successively, until desired polymer layers are obtained which are efficient to increase the Residual Resistance Factor RRF, the control of pressure as well as injection flow rate of the various slugs of aqueous polymer solutions so as to indicate if the desired increase in RRF has been attained;
by injecting a further water slug, displacing, after the addition of the polymer slugs, the last polymer slug the farthest possible from the face of the well;
in order to create oil saturation before the well, injecting a low volume-slug of organic solvent (either an aliphatic solvent or the formation oil itself);
shutting the well for 24 hours to cause the polymer to adsorb to the rock; and
resuming well production.
Thus the present invention provides a process for the selective reduction of the relative permeability to water in formations having wide range of salinity and permeability, said process being based on the injection, into a production well, of a slug of an aqueous solution of a cationic (or anionic) polymer, forming a layer, to which are adsorbed successive layers of anionic polymer (or cationic and anionic polymer) and crosslinking agent. This is the so-called SELEPOL process.
The present invention provides still a process for the selective reduction of the relative permeability to water where the polymer layers are spaced by slugs of aqueous solutions to which an alkaline salt has been added, such slugs serving to: i) displace the slugs of polymer solutions away from the face of the well; and ii) separate the various polymer slugs, so as to avoid the straight contact between polymers of opposite polarities which would tend to flocculate and form gels having the ability to block the formation, such gels blocking the passage of any reservoir fluids, water, oil and gas.
The present invention provides further a process for the selective reduction of the relative permeability to water where the slugs of aqueous solution between the polymer slugs cause that the reactions between polymer slugs only occur between the polymers embedded within the formed polymer film and the polymers of the following slug, so that the film formation occurs in layers having a point-to-point instead of a tridimensional crosslinking, that is, there are no gels nor precipitates.
The present invention provides still a process for the selective reduction of the relative permeability to water where the absence of polymer gel makes that the permeability to oil remains practically unaltered relative to the situation where no polymer slugs are added.
The present invention provides still an extremely flexible process for reducing the relative permeability to water by using solely commercial products, this rendering such process cheap and versatile, since the need of each formation is supplied by the variation of the successive layers of a certain type of polymer. Still, the flexibility of the applications according to the present invention is caused by the varied molecular weights and hydrolysis degree of the commercial products used in the process, which will be dimensioned as a function of the reservoir conditions (kind of rock, temperature, salinity and permeability).